Electronic systems detect arriving radio frequency or radar waves. It is desired not only to determine the nature of a threat, but also to react to that threat. To do so, the angle of arrival of the radio frequency or radar signal must be determined. All of the parameters of the emitting apparatus, its position is the most difficult and slowest to change.
Systems producing information about the nature of the arriving signal and its direction of arrival are called "Electronic Support Measures" systems, and are designated herein as "ESM" systems.
In an ESM system, it is usual to use an omni-directional antenna to acquire the arriving signal. When a threat-signal is identified, a specific set of analysis-receivers is assigned to receive the signal and to evaluate the threat. Among other parameters angle of arrival of the threatsignal is desired.
ESM antennas include single beam, multibeam, scanning, monopolar, and interferometric direction fining antennas. The directional accuracy and angular resolution depends among others, upon the type of receiver used, the number and type of antennas in the array, the spacing between the antennas of the array, the field of view that must be intercepted by one antenna array, the number of arrays needed for total coverage, the ability of the receiver to preserve and track the phases of the antenna signals induced by the arriving signal, the signal-to-noise ratio at the phase-measuring point in the receiver(s), the accuracy of phase measurement in the receiver(s), and the receiver data-conversion characteristics used to derive the angle of arrival signal from the electrical phase differences among antenna signals.